There are currently many different radio standards in existence in North America and elsewhere. For example, in North America the frequency band 824-894 MHz (824 to 849 MHz for handset transmit and 869 to 894 MHz for handset receive) is reserved for cellular communication systems among which are AMPS (Analog Mobile Phone System) analog cellular defined by the standard EIA/TIA-553 (Electronic Industry Association/Telecommunications Industry Association Standard 553), TDMA (Time Division Multiple Access) digital cellular defined by the standard EIA/TIA/IS-136 (where IS means Interim Standard) and CDMA (Code Division Multiple Access) digital cellular defined by the standard EIA/TIA/IS-95. The frequency band 1850-1990 MHz (1850 to 1910 MHz for handset transmit and 1930 to 1990 MHz for handset receive) is, on the other hand, reserved for PCS (Personal Communication System) and the three main standards operating in this band are J-Std-009 which defines upband TDMA, J-Std-008 which defines upband CDMA, and J-Std-007 which defines PCS1900. PCS1900 is based on the European GSM (global System for Mobile), but operates in the North American PCS band.
A radio receiver designed to operate in compliance with one of the standards is not generally able to operate with any of the other standards. The fact that the cellular systems generally all have different advantages and disadvantages make it difficult for customers to gain all their desired properties in a single portable handset using a single cellular standard. For example, AMPS has an extremely large coverage area in North America while the coverage area for PCS1900 currently is limited to high population areas and high traffic highways. On the other hand, users of the PCS1900 system gain advanced features such as call display, secure communications, longer handset battery life, and a greater cost efficiency compared to the AMPS users.
To allow customers to increase the flexibility between cellular standards, a great deal of design research has been conducted to allow a single portable handset to operate within a plurality of cellular standards, such handsets are hereinafter referred to as multi-mode handsets. An example of a desirable multi-mode handset is one which is capable of operating in both the PCS1900 and AMPS networks, therefore gaining the added benefits of the PCS system while allowing the use of the AMPS network while communicating out of the PCS1900 coverage area. The operation of such a handset can be described as being dual-band/dual-mode since it must operate at two different frequency bands and also in two different modes.
For example, Nokia Mobile Phones Limited of Salo, Finland has designed a multi-mode handset with Nokia product number 6190 that allows the user to switch from the PCS1900 network to the AMPS network with the addition of an AMPS module. The user of the telephone removes the battery from the original handset, adds the AMPS module, and then reattaches the battery. At that point, the user can initiate a cellular call within the AMPS standard. Beyond the obvious disadvantage of an additional cumbersome module to carry around, this implementation makes it impossible for a user of the multi-mode handset to easily transfer from one cellular standard to another when the phone is in an activated mode.
There are currently PCS1900/AMPS multi-mode handsets that do not require the use of such cumbersome add-on modules. One key advantage of these new portable phones is the possibility of allowing a hand-over of an active call between different cellular standards to take place. Unfortunately, there is currently no system interaction between such networks as PCS1900 and AMPS. Thus while a person can place or receive a call on either system if they have a multi-mode handset, there is no facility for handing over an active call from one system to the other. The fundamental problem with such a hand-over is the complexity involved in coordinating two completely different radio systems which are owned and operated by different competing companies. To establish an effective and efficient hand-over procedure at the network and/or base station level, an increase in cooperation between these companies is required. Such cooperation presently does not seem likely in the near future.
A procedure is thus needed that can allow for the hand-over between competing cellular standards without requiring additional network cooperation or infrastructure. Such a solution must therefore be capable of implementation fully within a suitable multi-mode handset.